This invention relates to a shrouded omni-directional wind turbine which discharges vertically and is capable of extracting higher quantities of electric power than a free wind turbine of equal diameter.
The exponentially rising global demand for electric power and the significant and entrenched damage caused to the ecosystem through the generation of such power utilizing non-renewable fuels such as oil and coal, together with the rapid depletion of these resources and the lack of other natural resources to keep up with growing demand, has in the recent past provided new impetus to look towards the further development of renewable energy sources.
Mankind has attempted to utilize the vast amount of power available in the wind, for over several centuries and has been successful in powering sailing ships, pumping water and grinding grain. Although, since the rotating electric generator was invented some attempts have been made to utilize wind power to drive generators, it is only in the last 50 years, with the discovery of strong and light weight materials; wind power has begun to be considered economically viable for this purpose.
Wind turbines can be broadly divided into two groups. The “horizontal” types, as in the very familiar Dutch windmill and the “vertical” types, as in the wind speed measuring cup/paddle or Darrieus airfoil unit. Although, “vertical” wind machines are well known for their simplicity of design, strength and fewer moving parts, due to the fact that they need not be constantly rotated to face the wind direction, their lower efficiencies in comparison to the horizontal type units has resulted in the horizontal type units being favored.
The commonly recognized theoretical analysis of wind power production indicates that, the power extractable from the wind is in proportion to the intercepted wind area and the cube of the wind velocity. For wind turbines operating in free wind conditions, only by increasing the blade diameters to sweep larger areas can more power be extracted from the wind. This can now be seen in commercial power supply units having blade diameters in excess of 150 meters. According to Betz's Law the theoretical maximum level of power extraction is limited to no more than 59% of that available in a square meter of free wind. However, even with today's high-tech machines this level is far from being achieved at present.
An alternative approach has been to utilize diffusers, shrouds or other devices to accelerate the free wind to increase its energy density per square meter, prior to the wind reaching the rotor blades. As the energy extractable is proportional to the third power of the wind velocity, even minor accelerations can lead to significant increases in energy density and thus extractable power. This also enables the power extraction to commence at much lower wind velocities and be available for significantly longer periods during the year. Consequently these could be utilized in areas where the wind velocity is lower than that useable by the free wind turbines.
Regardless of these advantages, the fact that a large shroud type structure needs to be flexibly mounted in a highly wind exposed position and be rotatable to face the wind has been a major draw back of such devices. Also, as the free wind speed increases, the magnification through the use of the shroud elevates the rotor speed to extremely high levels with consequent high stress levels on the rotor blades.
Vertically discharging shrouds which accelerate wind and containing wind turbines located in the vertical section have been attempted in an effort to combine the best of both applications.
These are mainly of two different categories. First category consists of units which, create a cyclonic action through spiral air movement formation (vortex) utilizing the free wind, to either generate a pressure differential for suction of the air through a throat or directly impinge on the rotor to produce thrust for rotation. The second category consists of units through which the air movement is substantially irrotational. They rely on the free wind being accelerated and transported via concentric or segmented channels from the periphery of a cylindrical unit, to be discharged to a selected segment of the swept area of the rotor. The rotors used in both of these category units range from axial aerofoil type to mixed flow or centrifugal types. The first category offered much promise as it has the capacity for augmentation effects of well over ten fold but as yet these have not met commercial realization. The second category has limited augmentation effects as it relies on the acceleration of the free wind purely through direct concentration of the wind stream to increase its energy density. The concentration is achieved by reducing the cross section area through which the wind passes. This direct concentration has a maximum limit capability, as in an open environment the wind will simply by pass (‘leak’) any constricting device.
The main draw backs of these devices have been the significant cost of construction, due to complex shape requirements, high number of moving parts, high cyclic loading on the rotors as they crossed from active segments to inactive segments in their swept area, the leakage losses through inactive sections and the significant energy losses in forcing the free wind to move in a highly constricted manner through the augmenting devices. The high levels of constrictions generate resistance and energy losses which need to be over come by the free wind utilizing some of its energy. In many instances these resistances tend to reduce the quantity of wind passing through the device to such low levels that the augmentation could indeed be significantly negative. Many of them also rely on additional physical air ‘gates’ to prevent loss of air through inactive sections. Thus even if other benefits were possible; the additional costs of any augmenting device cannot be justified. As a consequence, augmented vertical-axis machines (even horizontal units) have not been commercially attractive and have not achieved acceptance in competition with the present form of horizontal-axis wind turbines.
However, because of opposition from environmentalists with regard to, injury to migrating bird life as well as prohibition by regulating authorities sighting intolerable low frequency noise, stroboscopic light reflection effects and safety hazards associated with these large propeller machines in populated areas, many areas which would be ideal for generating wind energy, such as atop large buildings, are simply off-limits. Due to the above these machines, are remotely located significantly away from the area of power usage, thus necessitating construction of expensive power grids to transport the energy produced to the consumers, generally living in large urban areas. Consequently, there is a further reduction in available power due to losses in the transmission grid and increased costs of transmission.
In addition these machines are also not capable of being located in areas where, high levels of wind resource may be available but which experience frequent change in wind direction due to the local terrain or have periods of highly elevated wind velocities such as cyclones or twisters, as these may easily damage the fully exposed rotor blades.
It is an object of the present invention to address or at least ameliorate some of the above disadvantages.